Marine seismic operations utilizing multiple air guns require means for precisely synchronizing the release of energy from each gun in order to optimize the composite acoustic pulse. A primary requirement of any synchronizing control system is means for detecting the exact time at which air is released from each gun for forming a seismic acoustical energy pulse.
The high pressures and adverse mechanical operating conditions prevailing in the proximity of marine air gun energy sources to delineate the sedimentary section below greatly reduce the reliability of conventional transducers such as hydrophones, accelerometers or pressure detecting devices which might be employed to detect the instant of energy release for recording on the seismic record, such as the acoustical to electrical converter disclosed in U.S. Pat. No. 3,496,532.
We have invented a practical and reliable means for detecting the air released from the guns or the instantaneous beginning of the seismic energy pulse.
The electrical event which results from circuitry arranged to detect this instant and momentary change of resistance is essentially free from uncertainty and mechanical noise which normally affects conventional transducers.
Conventional air guns used as a submerged seismic energy source are illustrated in U.S. Pat. Nos. 3,379,273, 3,653,460, 3,923,122 and 4,141,431. Likewise, in a typical synchronized seismic exploration system as illustrated in U.S. Pat. No. 3,496,532, a seismic stream cable is towed through the water to detect the acoustic reflection waves in the water generated by the detonation of the charge for converting the received reflected acoustic waves and convert them into corresponding electrical signals for the recording and processing apparatus. Thus, in the present day equipment, the time lost, as the few milliseconds travel time through the water between the air gun and the weaving towed sound detector a few yards or meters away has to be tolerated. Since typical velocity of sound in water is 5,000 ft./sec., the travel time to a detector towed, say 10 feet away from an air gun would be 2 milliseconds.
In systems like that disclosed in U.S. Pat. No. 4,141,431, plating takes place on one of the electrodes after several firings of the air gun, and cross coupling results between channels. Also, a fail-safe system is lacking in the above patent since if either electrode became defective, the system would fail to operate.